This invention is in a process for recovery of copper and the optional recovery of silver and gold from sulfide- and oxide-containing materials, which contain said metals, and are leached with water-soluble cyanides.
It is known from British Patent Specification No. 1,050,303 that sulfide copper ores, such as chalcocite (Cu.sub.2 S) or oxide copper ores, such as malachite (Cu.sub.2 (OH).sub.2 CO.sub.3) or azurite (CuCO.sub.3.Cu(OH).sub.2) in an aqueous dispersion can be treated under alkaline conditions with water-soluble cyanides, such as sodium cyanide, whereafter the copper is complex form, e.g., as sodium-copper cyanide (Na.sub.2 Cu(CN).sub.3) can be dissolved out of the solids. Water-soluble sulfide compounds, such as sulfides or hydrogen sulfides of alkalies or alkaline earths, are added to the cyanide solutions which have been removed from the solids. The metal sulfide, such as copper sulfide and silver sulfide, is quickly precipitated after the pH value has been adjusted to below 5 by the addition of sulfuric acid. The precipitate is filtered off and the free hydrocyanic acid is recovered from the filtrate by introducing a stream of gas, such as air or steam, into the filtrate to strip the acid. The acid vapor is either condensed or introduced into alkaline solutions and is re-used. This process is generally carried out at ambient temperatures and under atmospheric pressure. With the exception of the recovery of hydrocyanic acid, the process can allegedly also be carried out under superatmospheric pressure although this is not very economical.
In the known process, the hydrocyanic acid must be stripped by means of gas streams at high flow rates and in a second step must be converted to alkali cyanide for re-use. Such a process is very expensive regarding capital investment and operating costs.